Never really gave it much thought because once I get the back stab I'm instantly focused on grappling/walking away + I use the eternal reward so I don't really see the body, but I played a bit to see my favourite and I ended up picking demoman.

For your primary it depends on your skill level. If you are able to get headshots, use the heatmaker because it's much quieter and therefore tougher to pinpoint the location of, and occasionally the boost is actually very helpful if you can remember to use it.
For the secondary I personally use...

Demo: It's fun to bait people into your sticky traps, and I love getting chains of multiple direct hits in a row, and to get many kills with a single bomb.
Engineer: I love using shotguns, and with 150 hp (gunslinger) and average speed + a short circuit, it's very strong as with enough skill...